DESCRIPTION (Taken from the applicant's Abstract) Biologic effects of 1,25-dihydroxyvitamin D3 (1,25(0H)2D3) are mediated via genomic and non-genomic mechanisms. Although genomic mechanisms play a role in 1,25(0H)2D3-mediated hypercalcemia, non-genomic mechanisms, characterized by increased gastrointestinal absorption of Ca2+, are probably more important. Serum and urinary calcium levels and other markers of bone turnover are routinely used to monitor patients on 1,25(0H)2D3 therapy because hypercalcemia is the dose limiting toxicity and tests for calcium measurement are readily available at most clinical centers. However, hypercalcemia and other bone turnover markers are unsuitable as specific markers because of the dual nature of the mechanisms involved and genomic effects of 1,25(0H)2D3 do not correlate with its hypercalcemic effects. The anti-proliferative, differentiation and apoptosis-induction properties of 1,25(0H)2D3 are due to genomic responses, which can be exploited to treat human diseases characterized by cellular hyperproliferation as seen in cancer. This has led to the need to identify specific markers of genomic action of 1,25(0H)2D3 in humans. This project describes studies whose specific aims are to determine if peripheral blood monocytes can be used as a surrogate tissue in the assessment of genomic effects of 1,25(0H)2D3 in patients receiving doses in excess of 4 micrograms/day. Monocytes are suitable candidates for these studies because they are highly sensitive to the genomic action of 1,25(0H)2D3, are readily purified from other peripheral blood mononuclear cells, and can be sampled repeatedly using relatively simple, non-invasive techniques. The genomic action in blood monocytes will be assessed by measuring mRNA and enzyme activities of well-known 1,25(0H)2D3-responsive genes: 24-hydroxylase, cytidine deaminase, fructose-1,6-diphosphatase, alkaline phosphatase and lipoxygenases, in addition to VDR (a vitamin D receptor) and p21 and related to plasma levels of parent drug and metabolites (measured by high performance liquid chromatography). The pleiotropic nature of the genomic action of 1,25(0H)2D3, together with the relative lack of specificity of these enzyme activities, results in different time courses of multiple markers in their expression relative to drug administration. These pharmacokinetic:pharmacodynamic studies will provide new insight into the effects of 1,25(0H)2D3.